1. Field of the Invention
The present invention relates to an internal focus objective lens. Specifically, the present invention relates to an objective lens that is used with an astronomical telescope, terrestrial telescope, photo lens, and the like. The focusing on near objects is accomplished by moving a lens group that forms part of the objective lens.
2. Description of Related Art
Internal focus objective lenses are known as disclosed by Japanese Laid-Open Patent Application No. 51-32327. U.S. Pat. No. 4,027,935 to Momiyama is a U.S. equivalent of the Japanese application. The objective lens of the Japanese application is a telephoto-type objective lens. It includes, in order from the object side, a first lens group with a positive refractory power and a second lens group with a negative refractory power. Generally, a telephoto-type objective lens has the ability to have a lens length, defined as the distance from the optical system surface closest to the object to the image surface, shorter than focal distance of the lens system. In addition, the objective lens is an internal focus-type lens. Internal focus objective lenses carry out focusing on close objects by moving a second lens system along the optical axis toward the image side. Further, the optical system can be made waterproof by the above arrangement where only the second lens system moves in the lens system's housing.
However, with the conventional internal focus objective lens described above, the overall length of the objective lens is long. This arrangement does not permit the optical system to be compact. In addition, since the distance that the second or focus lens group moves is long to focus on close objects, it is difficult to properly correct the various aberrations.